Download Properties of Transmission

MODULO 9 “OUR WIRED WORLD”
TYPES OF NETWORKS:networks can be classified in relation to several features: the
distance they cover, the topology they have, the types of connections they use, the types
and methods of transmission and so on.
DISTANCE:
LAN (the acronym stands for “Local Area Network)
-it can cover only small distances (generally a building)
-it can connect a few computers or several hundreds
-usually privately controlled by local administration
MAN(“Metropolitan Area Network)
-it connects two or more LANs in the same city (usually covers a few kilometres)
Used by banks, libraries, schools to connect branches
WAN (“Wide Area Network)
-they connect several LANs over big distances (a country or the entire world)
The Internet is the largest WAN
INTRANETS:
-it is a private network, users must have permissions and passwords to access the Intranet
of an organization and to access sensitive data, generally protected by firewall
EXTRANETS:
- it is halfway between an Intranet and the Internet: it is similar to an Intranet because it is
private but it allows different degrees of access to different individuals using passwords.
(ex. A company might have an Intranet for its employees and an Extranet for business
partners)
NETWORKING DEVICES
Networking devices are necessary to link a computer to a network or several networks
among themselves.
They can be divided into:
1) End-User devices (or host devices)
-examples of host devices are: computers, printers, scanners, they can exist without
a network but in this case their capabilities are reduced.
Host-devices are connected to networks through:
NIC (Network Interface Card): it connects a host to a network
-consists of a printed circuit board that is plugged into an expansion slot in the
system unit. (also called network adapter)
-each NIC has a unique address called MAC (Media Access Control) this address
controls the transfer of data from the host to the network and vice versa
2) Network Devices
These devices allow the transfer of data among host devices in a network. They
can be classified into:
Repeaters: they amplify and regenerate signals along a network
Hubs: they concentrate connections (many hosts are seen as one)
Bridges: hardware and software resources which allow devices on a network to
communicate with devices on another network. A bridge also performs a check on
the data before it allows it to cross the network
A bridge can also convert data formats
Switches:they transfer data only to the connection that needs that particular data
but (unlike bridges) a switch can’t convert data formats
Routers:they have all the functions of other devices (regenerate signals,
concentrate connections etc) BUT they also regulate traffic on the network and
choose the most efficient route for each packet.
TOPOLOGIES
The topology of a network concerns the network’s layout or architecture that is
how the different nodes are connected to each other.
Topologies can be Physical or Logical
Logical Topologies: concern HOW the hosts communicate
Broadcast: each host sends its data to all the other hosts, the stations do not have
to follow a particular order to use the network (“first come first serve basis”),
Ethernet works in this way
Token passing: -network access is controlled by a Token which is passed
sequentially to each host, host can transmit only when it has got the token (ex.
Token Ring)
Physical Topologies: concern HOW the hosts are connected
BUS: all hosts are connected to a central cable in a straight line Adv. Easy to set up, quite
cheap, Disadv. Collisions (all that travels on same cable), if main cable breaks ,entire
network is disrupted
RING: each host is connected to the next and the first to the last creating a physical ring
Adv. Easy to set up, few collisions (token ring transmission) DISADV: slow (packets
must travel around entire ring to reach destination), if main cable breaks communication is
generally disrupted
STAR: all nodes are connected to a central point of concentration; ADV. No collisions, if
one cable breaks network continues to work, DIS. If central connection fails, network can’t
transmit
EXTENDED STAR: several star topologies connected to a central node that controls
everything. ADV.DIS. (same as for star topology) + extended star can extend size of a
network
MESH: nodes are connected with many interconnections; ADV: provide the best
protection from interruption of service (ex. Nuclear stations), DIS.difficult and expensive to
set up
WIRE AND WIRELESS MEDIA
Wire media: use cables to allow the connection, three types of physical links:
Twisted pair: two wires twisted together so that they don’t pick up interferences, used
mainly by telephone system
Coaxial cable: two wires: one made of fine strands of copper woven together to form a
tube, second inner tube of solid copper, space between two wires filled by non -conducting
material, quite expensive, used to be popular to link computers in offices
Fibre-optic cables: hundreds of very thin glass or plastic fibre strands, cheap, light,
secure, allows fast transmission, lasts a long time
Wireless media: connections are established using electromagnetic energy that travels
through space
Infrared:transmits data as light rays, can cover only short distances, line of sight must be
clear ex. Remote controls for TV, keyboard and mouse
Radio frequency: uses radio frequencies of electromagnetic radiation to transmit, can
function even if line of sight is interrupted, expensive, generally used for cellular phones
not computers, other sources of electromagnetic radiation can interfere with radio
frequency
Microwave: uses microwaves and satellite links to establish communication even with
distant locations, atmospheric conditions can interfere
Properties of Transmission
Concerns data movement: simplex vs. duplex, SIMPLEX: it is a one-way transmission
ex. display terminals at airport), DUPLEX: data can travel in two directions, half-duplex if
data flows in both directions but not at the same time, full-duplex if data travels in both
directions at the same time
Unit of measurement for data transmission is BPS (bits per second)
ADSL: asymmetrical digital subscriber line asymmetrical because more data travels
downstream than upstream
Methods of Transmission
Asynchronous: data transmitted one byte at the time, each byte precede by a start bit to
alert receiving computer, bytes are not connected together so message is reassembled
before it can be read, cheaper
Synchronous: data transmitted in blocks of characters , source and destination nodes
operate in synchronization so very fast transmission but more expensive